Camera, always buy, never rent
My reasoning is that your schedule becomes much more flexible...and using my math that I've been advocating, time=money...if you can't afford the money, spend the time...if you rent, you don't have the time as it costs money.
I own my camera, I've easily paid for it by now just on saved rentals alone. 8-9 shorts (1 weekend each) and a feature film (8 Weekends to shoot). I wouldn't have been able to do any of them as I had the money for the camera once...and never again. I've spent less than $1000 on all of the projects combined.
I do see that at a certain level of your filmmaking career (budgeted work) you may consider renting...but I am not a proponent of that as I see more flexibility and long term cost savings in ownership.
My camera is a 6 year old camera and works great so long as the majority of my distribution is still on SD...which it is. HD is still a smallish market comparatively. Once it gets bigger, I'll re-invest in a new camera that will last for a long time, like the RED...or the JVC HD100/200 series (budget dependent at that time, but I'd rather have DoF control that actually looks like a 35mm camera than an 8mm (1/3" chip) or 16mm (2/3" chip) camera. The relationship between the defocussed backgrounds and relative size to the subjects is different between the 3 of them. I really like the 35mm look (old photographer, can't help it). Besides, I can rent myself out to help recoup the cost of buying the RED.
Ownership also gives you familiarity so you can shoot faster, unless you're going to keep renting the same camera every time you shoot (at which point you've negated the argument that you rent to stay up to date).
If you plan to make lots of movies, I don't see a good business reason not to own the camera.
